Wireless communication devices, such as, but not limited to, wireless telephones, use many electronic components to transmit and receive signals over the air. A transceiver is the part of a wireless telephone that actually sends and receives signals. The front end of a transceiver is the portion of a transceiver closest to the air interface in the signal path. The front end includes an antenna and several components near the antenna in the signal path. Several of the components required in the front end of the transceiver are power amplifiers (PA's), isolators, low noise amplifiers (LNA's) and multiplexers. Each of these components are typically manufactured as packaged devices. In the case of a PA or an LNA, this package typically includes the active device and internal input and output matching circuits for bringing the input and output resistances up to an industry standard 50 ohms.
In one common embodiment, the packaged PA is comprised of a high performance FET (e.g., GaAs) placed on a ceramic or other substrate. Other active devices can be used, such as, for example, bipolar junction transistors (BJT's) and high electron mobility transistors (HEMT's). The matching circuits may be patterned on the ceramic substrate, or they may be fabricated using lumped surface mount technology (SMT) components. The FET is bonded to the package substrate, possibly to a metal heat sink, then typically connected to its input, output and bias pads using bond wires.
Depending on the requirements, multi-stage PA devices may be used as well. This means that one PA device may include more than one amplifying transistor. This may be necessary for a number of reasons. One possible reason is to produce the required gain. In the case of a multi-stage PA device, inter-stage impedance matching circuits may be used as well, to match between the output of one stage and the input of the following stage.
The inputs, outputs and bias lines to the FET are routed down to the ceramic substrate. After passing through the matching circuits, the input and output lines are routed off of the substrate down to the underlying printed wire board (made of FR-4 in most cases) through connectors on the PA package. Further wire bonding may be required to connect the package pads to the input, output and bias lines.
The package further comprises some kind of packaging (typically polymer) encasing, in whole or in part, the FET and the ceramic substrate holding the matching circuits. The input and output bias leads can be found at the edge of the packaging.
Isolators, duplexers, diplexers and low noise amplifiers (LNA's) are handled in much the same way. As packaged devices, they each have their separate substrates with their separate matching circuits bringing their input and output impedances to 50 ohms.
Most RF test equipment can only test parts at an impedance of about 50 ohms. Manufacturers and designers typically want to be able to test each part separately. Historically, the only way this could be done was if each part had input and output impedances of around 50 ohms. For this reason, parts, such as PA's and LNA's, for example, have typically been manufactured with impedances equal to about 50 ohms. This has required the use of extensive input and output matching circuits for many of these parts.
A duplexer is one of the primary components in a transceiver front end. The duplexer has three ports (a port is an input or an output). One port is coupled to an antenna. A second port is coupled to the transmit signal path of the transceiver. The duplexer couples the transmit path to the antenna, so that the transmit signal can be transmitted on the antenna.
A third port is coupled to the receive path of the transceiver. The antenna coupled the antenna to the receive path, so that the received signal can be received by the receive path of the transceiver.
An important function of the duplexer is to isolate the transmit signal from the receive path of the transceiver. The transmit signal is typically much stronger than the receive signal. Some of the transmit signal inherently gets down the receive path. But this transmit signal going down the receive path must be greatly reduced (or attenuated). Otherwise, the transmit signal going down the receive path will swamp, or overwhelm, the receive signal. Then the wireless telephone will not be able to identify and decode the receive signal for the user.
The required attenuation of the transmit signal going down the receive path is achieved at some expense. The duplexer also attenuates the transmit signal going to the antenna for transmission. This attenuation in the transmit signal going to the antenna is known as loss. It would be beneficial to reduce the transmit path loss in the duplexer.
Additionally, the duplexer typically must be large accomplish the receive path attenuation of the transmit signal. Consumers are continually demanding smaller and smaller wireless telephones with more and more features and better performance. Thus, it would be beneficial to reduce the size of the duplexer while maintaining or improving the transmit signal attenuation in the receive path and simultaneously maintaining or improving the transmit signal loss to the antenna.